1. Field of the Invention
The present invention relates to a rolling contact parts steel for use in bearings used, e.g., with automotive vehicles, agricultural machinery, construction machinery and iron-and-steel machinery, and to bearings including races (an outer race and inner race) and a rolling element at least one of which is made of the rolling contact parts steel and, more particularly, to a long life rolling contact parts steel and long life rolling bearings required for use in transmissions and engines.
2. Description of the Related Art
A rolling bearing is typically in a severe service under high surface pressure so as to experience a repeated shearing stress. In order to endure the repeated shearing stress and to secure a long rolling fatigue life (hereinafter referred to as "life", a rolling bearing with a rolling contact part which is made of high-carbon chromium bearing steel (e.g., JIS-SUJ2), then quenched and then tempered so as to have a 30-48 H.sub.R C hardness has been used.
On the other hand, there is a prior-art rolling bearing made of case hardened steel in order to increase its life. This prior-art case hardened steel rolling bearing has been made of a case hardened low-carbon steel of a good hardenability, e.g., JIS-SCR420H, -SCM420H, -SAE8620H or -SAE4320H since a hardness curve of the case hardened steel rolling bearing must be determined in accordance with an internal shearing stress distribution created by a contact surface pressure. Then, this case hardened steel rolling bearing has been sequentially carburized or carbonitrided quenched and tempered and thus has secured a required life so that the races and rolling elements of the case hardened steel rolling bearing have a 58-64 H.sub.R C surface or case hardness and a 30-48 H.sub.R C core hardness.
On the other hand, e.g., Unexamined Japanese Patent application publication No. SHO 49-114516 disclosed a prior-art bearing material. This publication provides a carburized medium-carbon steel for a rolling contact part including C: 0.36-0.50 wt % and Cr: 0.5-1.5 wt % etc., and teaches that in accordance with this carburized steel used for rolling elements, reducing a carburization time and the content of retained austenite in the carburized surface of the rolling elements increases the tenacity, hardness and fatigue limit of the case hardened rolling elements.
A typical load on a machine using a rolling bearing and the rotational speed of the machine have increased so that service conditions of the rolling bearing has become more severe, and the rolling bearing has been used under a quasi-high temperature to a high temperature, which have caused the following problems:
First, an increase in the service temperature of a rolling bearing decreases the hardness of the rolling bearing, resulting in a plastic deformation in the rolling bearing which decreases the life of the rolling bearing. Second, using a rolling bearing under high temperature can break an oil film between the races and the rolling elements of the rolling bearing so that a boundary lubrication is ready to occur which accelerates wear in the rolling contact parts of the rolling bearing. Third, even when the content of retained austenite present in the carburized surface is reduced, even a slight content of retained austenite present in the surface of a rolling contact part transforms to martensite under quasi-high to high temperatures so that the size of the rolling contact part changes which damages the dimensional stability of the rolling bearing. In particular, a severe dimensional stability of the rolling bearing has been recently required. For example, a slight dimensional error in a rolling bearing used in a jet engine or the like can cause a fatal defect. Therefore, the rolling bearing in service under quasi-high to high temperatures requires a severe dimensional stability.
Thus, in order to preferentially secure the dimensional stability of the rolling bearing in service under quasi-high to high temperatures, a high-temperature tempered type of high-carbon chromium bearing steel (e.g., JIS-SUJ2) in which the case hardened high-carbon chromium bearing steel has been high-temperature tempered so as to previously transform retained austenite in the rolling contact part of the rolling bearing to martensite and thereby increase the dimensional stability of the rolling contact part has been conventionally provided.
Precipitation-hardened steel, such as M50, high-temperature bearing high-speed steel with Cr, Mo and V, has been provided as a material for quasi-high to high temperatures rolling contact parts in addition to the high-temperature tempered type of JIS-SUJ2 steel. M50 is a high-temperature bearing high-speed steel which is high-temperature tempered to precipitate an alloy carbide so as to provide a sufficient high-temperature service strength to a rolling bearing.
In addition, in order to increase the life of a rolling bearing, the service temperature of which can be a quasi-high temperature to a high temperature, a wear-resistant carburized steel of Unexamined Japanese patent application publication No. SHO 53-37518 or a case hardened steel of Unexamined Japanese patent application publication No. SHO 54-75420 can alternatively be used, for example, as a material for a rolling contact part.
The high-temperature tempered type of JIS-SUJ2 steel increases the dimensional stability of the rolling contact part while high-temperature tempering decreases the hardness of the rolling contact part and increases the plastic deformation in the rolling contact part made of the high-temperature tempered type of JIS-SUJ2 steel so as to decrease the life of a corresponding rolling bearing. In addition, the insufficient wear resistance of the high-temperature tempered type of JIS-SUJ2 steel can remarkably accelerate wear in the rolling contact part under a boundary lubrication under a high-temperature service of the rolling bearing.
In addition, since the concentration of carbon in M50 precipitation-hardened steel is high and M50 steel precipitation-hardened steel in the stage of material for rolling contact part has macrocarbides of Cr, Mo and V, the workability in the pretreatment of M50 steel is poor. On the other hand, the macrocarbides cause a stress concentration therearound which can result in a flaking from the place of the stress concentration to thereby decrease the life of a corresponding rolling bearing with a rolling contact part made of M50 steel. Thus, M50 steel must be additionally solution heat treated at a high temperature (about 1,100.degree. C.) in order to dissolve the macrocarbides into the matrix of M50 steel and, then, particularly heat treated in order to refine the resulting carbides. This requires separate heat treatment equipment and steps and decreases the productivity of the heat treatment.
The wear-resistant carburized steel of the Unexamined Japanese patent application publication No. SHO 53-37518 can produce macrocarbides which reduce the life of the rolling bearing. The carburized wear-resistant steel of the Unexamined Japanese patent application publication No. SHO 54-75420 can also produce macrocarbides and, in particular, is more likely to do so when the content of carbon in the carburized wear-resistant steel is large.
Conventionally, it has been known that an in-steel nonmetal inclusion, in particular, an oxide-base inclusion, deteriorates the mechanical property of a steel. The present inventors confirmed that the prior-art high-carbon chromium bearing steel, low-carbon alloy steel, case hardened steel, etc., failed to take into account a reduction of oxide-base inclusions so as to reduce the life of the rolling bearing.
Therefore, a primary object of the present invention is to provide a rolling contact parts steel which produces a fine carbide without the need for a separate heat treatment and which will not reduce the hardness of the parts even when a rolling contact part made of the rolling contact parts steel is high-temperature tempered in order to increase the dimensional stability of the rolling contact part and which produces a very slight content of an oxide-base inclusion.
Another object of the present invention is to provide a rolling bearing made of the inventive rolling contact parts steel which has a long life in service under not only room temperature but also quasi-high to high temperatures.
A further object of the present invention is to provide a rolling bearing which has a good dimensional stability even in service under high temperature.